3. Download the latest version of the OS for your device. There's a link at the top of the 8500 Forum for latest OS builds for each model. Make sure you download the OS for your exact model. That means NOT downloading the 8530 OS for your 8520.

5. Install Desktop Manager, then install the OS you downloaded for your 8520.

6. Update the OS on your wiped 8520, making sure to follow the instructions in the BBF FAQ: How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ
Make sure you delete the vendor.xml file as per the instructions. Failing to do this will prevent the update process from running. I've forgotten this step and was on the verge of pulling out my hair trying to figure out what was wrong. Just look at my profile/avatar pic!

7. When you reload your third-party apps, go slow. Usually this Error 523 is caused by an app that wasn't written properly, conflicts with another app, or just gags your BlackBerry.